1. Field of the Invention
The present invention relates to a method and a receiving unit for the detection of data symbols contained in a received radio signal. The invention relates further to a transmitting/receiving device and to an integrated circuit with a receiving unit of this type.
2. Description of the Background Art
The invention falls within the field of data transmission. Although it can be used in principle in any digital communication system, the present invention and its underlying problem will be explained below with reference to a “ZigBee” communication system in accordance with IEEE 802.15.4.
So-called “Wireless Personal Area Networks” (WPANs) can be used for the wireless transmission of information over relatively short distances (about 10 m). In contrast to “Wireless Local Area Networks” (WLANs), WPANs require little or even no infrastructure for data transmission, so that small, simple, power-efficient, and low-cost devices can be implemented for a wide range of applications.
Standard IEEE 802.15.4 specifies low-rate WPANs, which are suitable with raw data rates up to a maximum of 250 kb/s and stationary or mobile devices for applications in industrial monitoring and control, in sensor networks, in automation, and in the field of computer peripherals and for interactive games. In addition to a very simple and cost-effective implementability of devices, an extremely low power demand of devices is of critical importance for such applications. Thus, an objective of this standard is a battery life of several months to several years.
At the level of the physical layer, in the virtually globally available 2.4 GHz ISM band (industrial, scientific, medical) for raw data rates of fB=250 kbit/s, the IEEE standard 802.15.4 specifies a band spread (spreading) with a chip rate of fC=2 Mchip/s and an offset QPSK modulation (quadrature phase shift keying) with a symbol rate of fS=62.5 ksymbol/s.
In an 802.15.4 transmitter for the ISM band, the data stream to be transmitted is first transformed into a series of PN sequences (pseudo noise) with the use of four data bits in each symbol period (TS=1/fS=16 μs), in order to select a total of 16 PN sequences. Each symbol of four data bits is assigned in this manner a symbol value-specific PN sequence of 32 PN chips (chip period TC=TS/32=500 ns=1 fC), which is transmitted instead of the four data bits. The “quasi-orthogonal” PN sequences P0, P1, . . . , P15, specified in the standard, differ from one another in the cyclic shifts and/or inversion of every second chip value (see IEEE Standard 802.15.4-2003, Chapter 6.5.2.3).
The PN sequences allocated to the successive symbols are linked together and then offset QPSK modulated (quadrature phase shift keying) by modulating, with half-sine pulse shaping, the even-indexed PN chips (0, 2, 4, . . . ) onto the in-phase (I) carrier and the odd-indexed PN chips (1, 3, 5, . . . ) onto the quadrature-phase (Q) carrier. To form an offset, the quadrature-phase chips are delayed by one chip period TC with respect to the in-phase chips (see IEEE Standard 802.15.4-2003, Chapter 6.5.2.4).
Both coherent and incoherent approaches are known to detect data symbols contained in an incoming signal. Whereas in coherent approaches the incoming signal is converted into the complex envelope (baseband) by using a frequency- and phase-correct carrier wave and obtained from the carrier control circuit, in incoherent approaches at least the correctness of the phase, within limits possibly also the correctness of the frequency of the carrier wave, can be eliminated.
A coherent receiving unit is known from the textbook “Nachrichtenübertragung” [Message Transmission] by Karl-Dirk Kammeyer, second edition, B. G. Teubner, Stuttgart, ISBN 3-519-16142-7 (FIG. 12.1.7 on page 417). A disadvantage in this case is the high realization cost, which arises, on the one hand, from the necessary carrier control circuit with the associated high-rate (higher than the chip rate) multiplication of the incoming signal with the frequency- and phase-correct carrier wave and, on the other, from the costly and complex signal processing with a high-rate complex matched filtering. This high realization cost causes in addition a very high power consumption.
A method and system is disclosed in ADLER, Robbie: Adaptive Modulation and the IEEE 802.15.4 standard: Power Performance Tradeoffs, EE359 Project, Fall 2004; a modification of the “physical layers” (PHY) of the 802.15.4 is disclosed to the effect that chip sequences are introduced whose length, i.e., number of chips, changes adaptively.
Furthermore, an incoherent receiving unit is known from the indicated textbook (FIG. 12.3.7 on page 447) and from the third edition of the textbook “Nachrichtenübertragung” [Message Transmission] (Karl-Dirk Kammeyer, Teubner-Verlag, Wiesbaden, ISBN 3-519-26142-1). It has an FM discriminator, an integration unit, and a so-called limiter and requires the processing of high-rate (higher than the chip rate) and in part complex-valued signals.